Technical Field
This invention relates to fuel cell systems, and specifically to, fuel cell systems adapted to estimate output power that can be output by an involved fuel cell.
Background Techniques
There are known fuel cells employing electrochemical reactions for generating electric power. Such fuel cells are generally made up as a fuel cell stack composed of a set of stacked multiple minimal constituent units called unit cells. Unit cells have a structure including an anode electrode supplied with hydrogen, and a cathode electrode supplied with oxygen in the air, each respectively having at one side thereof a diffusion layer and a catalyst layer laminated thereon in order. The sides each having a diffusion layer and a catalyst layer laminated thereon are both set inside, with a central electrolyte interposed in between. The anode electrode and the cathode electrode have electric power output terminals thereof connected to an external circuit. The external circuit is connected to loads such as a motor.
In such the unit cell, hydrogen molecules supplied to the anode electrode are changed at the catalyst layer of the anode electrode to active hydrogen molecules, and further to hydrogen ions, discharging electrons. Such hydrogen ions are accompanied by moisture contained in the electrolyte, to move in this electrolyte, from the anode electrode toward the cathode electrode. Discharged electrons move to the cathode electrode, via the external circuit connected to the anode electrode. Such movements of electrons make electric currents conducted through electric loads connected to the external circuit. On the other hand, oxygen molecules supplied to the cathode electrode accept electrons in the catalyst layer, where electrons are moved from the external circuit, and are changed to oxygen ions to be combined with hydrogen ions moved thereto through the electrolyte, to produce water.
Such being the case, for the unit cell to have an outgoing electric current, the electrolyte needs moisture. As generally known, the electrolyte has an increased ionic conductivity, as it has a higher relative humidity. However, too much moisture gives rise to flooding phenomena impeding movements of hydrogen gases, constituting a difficulty to conduct electric currents. Such being the case, fuel cells tend to exhibit varied performances in power generation, depending on a degree of dryness of involved electrolytes.
Further, fuel cells tend to have lowered performances in power generation, under low temperature conditions such as those below the freezing point involving catalyst layers with lowered activities or electrolytes with lowered conductivities.
Such being the case, fuel cells have varied performances in power generation, depending on a dryness degree and a temperature of involved electrolytes. Therefore, when using a fuel cell for supplying electric power, one is unable to determine electric power that can be output from the fuel cell, having anxieties about un-preferable situations involving, for instance, conduction of an excessive electric current from the fuel cell causing, at the fuel cell, a reduced voltage to be output or a short in electric power to be output, resulting in non-conforming performances of loads.
For a fuel cell, the performance in power generation is defined as an output characteristic of the fuel cell. There has been a fuel cell system disclosed in Japanese Patent Application Publication No. 2009-99393 (referred herein to as a patent literature 1) including employing an output characteristic of a fuel cell determined from a temperature and an impedance of the fuel cell, as a basis to estimate electric power that can be output from the fuel cell.